Hybridization of plant and animal species has long been known to be a desirable means for improving the physical and productive qualities of crops and livestock. The benefits to be obtained from selective cross-breeding are manifold and are well known to agriculturalists. Some families of plants, however, are presently not conveniently susceptible to hybridization. These include plants such as cotton which are self-pollenating and in which sexual maturity arrives simultaneously for both the male and female sexual organs within a flower. In view of the high commercial utility and value of cotton to the agricultural community, means for the hybridization of cotton have long been sought.
Since the principal economic impact of cotton plants lies in the production of cotton fiber from bolls, it is desirable to produce hybrid cotton seeds capable of maturing into cotton plants having larger, more numerous, and more productive cotton bolls. It is similarly desired to produce cotton fiber having improved manufacturing and processing qualities.
Presently, the only system of achieving hybridization in cotton which is believed to have any significant commercial potential is a method known to those skilled in the art as the "cytoplasmic male sterility-restorer method." This technique has certain significant, inherent disadvantages including the lack of an adequate "restorer" system for genetic restoration of fertility in the hybrid.
Chemically induced male sterilization has been employed in some plant families to effect hybridization. Thus, rows or groups of plants are selected to become the female host plants for the production of hybrid seeds. These female hosts are rendered male sterile by contact with a selective chemical sterilant. Such rows or groups are interposed with rows or groups of plants selected to perform as male donor plants. Such male donors are not contacted with the chemical sterilant and produce pollen in the normal fashion. This pollen is allowed to contact the female sexual organs of the male sterile female hosts either through wind pollination, hand pollination or through the mediation of insects. If the female hosts are effectively male sterile, self-pollination of such hosts is precluded and uniform, exclusive pollination of such female hosts by the selected male donor plants is ensured. The seeds resulting from such cross-pollination are hybrid in nature and reflect differing male and female parental heritages. Such seeds, when planted and nurtured during subsequent growing cycles, may exhibit hybrid vigor, improved physical traits, and other benefits known to be associated with such hybrid genetics. See in this regard "Chemically Induced Male Sterility, a New Tool in Plant Breeding?" F. Wit. Euphytica Vol. 9, No. 1 p. 1 et. seq. (1960) and "Evaluation of Certain Chemicals as Selective Gametocides for Wheat", Porter et al. Crop Science p. 381 et. seq. (1961).
Chemical sterilants have heretofor been proposed for use in the hybridization of cotton. One such composition is Dalapon (Dow Company), believed to be a 2,2-dichloropropanoic acid. While Dalapon will promote male sterility in cotton plants, it also spurs vegetative growth of the plants at the expense of reproductive vigor. Cotton plants treated with this material exhibit low flower counts and develop few mature bolls thus evidencing diminished female fertility. Accordingly, Dalapon is considered to be commercially ineffective for use in the hybridization of cotton. A chemically similar material 2,3-dichloroisobutyrate (FW-450 of the Rohm and Haas Co.) which has also been reported to have gametocidic activity in cotton, is also believed to share shortcomings similar to Dalapon's. See "Effects of Sodium 2,3-dichloroisobutyrate on Six Characteristics of American Upland Cotton" Richmond, Crop Science, p. 58 (1961).
Pennwalt Corporation's TD 1123 plant growth regulator, which is potassium 3,4-dichloro-5-isothiazole carboxylate, has been suggested for use in late season termination for insect control in cotton and as a conditioner for defoliation. See "Termination of Late Season Cotton Fruiting with Plant Growth Regulators" Kittock et. al., Crop Science Vol. 17 No. 7 pp. 320-24 (1977) and U.S. Pat. No. 3,341,547 issued to Malley. It has also been suggested that TD 1123 may act as a selective male gametocide for cotton. See, for example, Olvey et al pre-conference brochure of the 1981 Beltwide Cotton Production Research Conferences and Special Meetings, p. 9. No method for attaining effective male sterility in cotton while maintaining good female fertility has been disclosed heretofore, however. Furthermore, no effective methods for the hybridization of cotton are known.